This invention relates generally to radio frequency circuits and more particularly to tuned radio frequency resonant circuits.
As is known in the art, radio frequency resonant circuits, such as a magnetically-tuned resonant circuit, are often used in radio frequency receivers. The magnetically-tuned resonant circuit generally includes a body of a ferrimagnetic material which in the presence of an external magnetic field provides the resonant circuit. A sphere comprising yttrium iron garnet (YIG) is often employed as the ferrimagnetic body. Generally in a so-called YIG filter, for example, two coupling loops, a first coupling loop disposed about an X-axis and a second coupling loop disposed about a Y-axis are provided with the YIG sphere being disposed within both loops. Each coupling loop is a conductor, shaped as a semicircle, with each coupling loop being disposed around a different portion of the YIG sphere. Generally, the coupling loops and the sphere are disposed in a body member commonly referred to as an RF structure. With a single crystal YIG sphere disposed in the RF structure and in the presence of suitably applied DC or steady magnetic field intensity H.sub.dc, such as applied between a pair of magnetically coupled pole pieces, the YIG body responds to an applied input RF signal fed to one of said coupling loops, if the input signal has a frequency component substantially equal to the resonant frequency f.sub.o of the resonant circuit provided by the YIG sphere and magnetic field passing through the sphere. The resonant frequency f.sub.o of such a sphere in a uniform resonant mode is given as f.sub.o =.gamma.H.sub.dc where f.sub.o is the centerband resonant frequency in the uniform resonant mode, .gamma. is a quantity which is a function of the material and which is generally referred to as the gyromagnetic ratio, and H.sub.dc is the magnitude of the applied DC magnetic field. Therefore, a portion of an RF input signal, fed to the input one of the afore-mentioned coupling loops, for example, the X axis coupling loop, is coupled through the YIG body to the output one of the coupling loops here the Y axis coupling loop if the portion of the input signal has a frequency component which equals the resonant frequency of the YIG circuit given by the relation f.sub.o =.gamma.H.sub.dc.
One problem associated with magnetically-tuned resonant circuits, such as the one described above, is the phenomenon generally referred to as microphonics. Microphonics is here a term which refers to the noise produced in an output signal in response to an externally applied mechanical force such as encountered during vibration of the YIG filter. The presence of a YIG filter in a vibrating environment provides external forces onto the YIG filter housing which provides small dynamic mechanical distortions in the filter housing, and concomitant therewith, changes in the magnetic permeability of the magnetically permeable portion of the YIG filter and hence the magnetic field strength H.sub.dc. Since the resonant frequency is a function of the field strength, the resonant frequency will also change in response to the external forces applied to the filter housing. In certain applications of a YIG filter, the resonant frequency of the filter must be maintained substantially at the desired resonant frequency independent of any mechanical vibration to which the YIG filter may be exposed. In some applications, this requirement is difficult to satisfy, particularly when the YIG filter is disposed in a vibrating environment such as is found, for example, in a guided missile.